1. Field of the Invention
The present invention relates to an x-ray tube. More specifically, the present invention relates to an x-ray tube configuration capable of generating high intensity x-rays that emanate from a small focal spot without a loss of reliability.
2. Description of the Related Art
X-ray tubes have a number of applications which involve the treatment or analysis of a sample, for example, industrial imaging, analytical instruments and medical imaging. For such applications, it is often desirable to have an x-ray tube which has a long service life, which is capable of forming a small focal spot and which is also capable of generating a high intensity of x-radiation at the sample.
X-ray tubes generally include an electron gun and an anode. A beam of electrons generated by the electron gun is focused to a focal spot on the anode, and x-rays are generated by the interaction of the beam of electrons with the atoms of the anode. These x-rays are generated in all directions emitting from the anode in the region surrounding the focal spot. Typically, the anode is substantially surrounded by an evacuated housing in which a window is formed to permit some of the x-rays to pass out of the housing, the window typically comprising a thin foil of a low atomic number metal, such as beryllium or aluminum, having a high transmission coefficient for x-radiation. Illustrative prior art x-ray tube structures in this regard are described, for example, in U.S. Pat. Nos. 5,751,784 and 5,563,923.
A commercially available x-ray tube 1 is schematically illustrated in FIG. 1 which employs a gun container 2 having a flat surfaced discharge end 3, i.e., gun snout, relative to the electron beam direction x, and x-rays emitted from anode 4 pass through an x-ray transmissive window 5 formed in a side of the hermetically sealed and evacuated envelope 6. The gun container 2 houses a cathode and filamentary heater used to generate an electron beam in a generally known manner. The gun snout 3 is flat in the sense it is a planar surface oriented perpendiculary to the beam direction x with a central aperture (not shown in the view) for transmission of the electron beam out of the gun container 2. Using the flat surfaced discharge end 3, focal spot sizes are yielded on the anode 4 (i.e., target) which tend to be relatively large, e.g., exceeding 20 microns in largest diameter, and which tend to be heavily limited by chromatic aberration. One approach to reducing the spot size is to reduce the working distance between the gun snout and anode. Smaller working distances produce smaller magnification for the beam; hence, a smaller spot at the anode. However, investigations conducted by the present inventors have shown that a flat gun snout tends to produce high field emission and instability when positioned nearer a target due to high electric fields. Consequently, such x-ray tubes with flat gun container snouts are focus limited due to chromatic aberration and instability issues, especially if attempts are made to position the gun assembly in closer proximity to the target.
It would be desirable to be able to position a gun snout in very close proximity to the target in order to reduce beam size and provide small spot focusing yet without unduly increasing the structural or operational complexity of the x-ray tube. An object of this invention is to provide an x-ray tube endowed with enhanced electron beam focusing performance and capabilities, and, more particularly, provide an x-ray tube with an electron gun positionable in close proximity to the target at reduced surface fields to achieve smaller focal spots without incurring instability.